In large (e.g., bulk storage) liquid tanks, and in particular tanks used to store hydrocarbons and other chemicals, at refineries, tank terminals, and the like, which are capable of holding large amounts of ‘oil’ products, use is frequently made of a floating roof, which floats on the liquid in the tank and is displaceable in a vertical direction. Such a floating roof is capable of following the level of a liquid (e.g., hydrocarbons) when the liquid is discharged from or filled into the tank. Floating roofs of this type are used for reducing evaporative losses (from a financial perspective) and also for preventing environmental contamination (e.g., in the USA-EPA regulations (so called Title V)). Such roofs are also employed to reduce explosion risks, and for other reasons, such as preventing ingress of, for example, rainwater from the surroundings into the liquid (e.g., in so called external floating roof tanks).
Typically, the prevention of evaporative losses (e.g., leakage) and ingress is enhanced by a sealing arrangement fitted along a perimeter of the floating roof for providing sealing and sliding contact with an inner wall of the tank. Further, using a roof that floats on the liquid enables minimizing the space between the liquid and the roof and thereby minimizing the amount of flammable gases in this space. In case of fuel and oil tanks, the environment on top of the floating roof is a hazardous or potentially hazardous environment.
Over the last several decades, severe accidents have occurred due to overfill of huge storage (e.g., oil, petroleum products, etc.) tanks (e.g., Buncefield accident in 2005 in the United Kingdom, and in Puerto Rico and Rajastan in 2009). Such accidents sometimes cause injuries, fatalities, and result in high costs to the company owning or managing the facilities where the storage tanks are located. Also, the company image is negatively affected. The impact on the environment can be enormous.
One of the reasons for such accidents is often due to the lack of an adequate overfill protection that is capable of detecting a high tank fill. Current protection systems are based on mechanical devices and are subject to failures and nuisance trips. Most of the systems also are designed for monitoring the liquid level only. Monitoring a floating roof has different requirements and poses different challenges.
Additionally, 30-40% of all storage tanks worldwide are equipped with a floating roof. The purpose of this floating roof is to reduce vapor emissions, both for environmental reasons (less volatile pollutions) and safety (reduction of explosion hazard), but also limiting loss of product. There is an international trend to provide all fixed roof storing lighter hydrocarbons with this type of floating roofs.
There are several concerns related to floating roofs. First, such roofs can become stuck, which might cause damage to the roof/tank, or in a worst case cause the roof to collapse and sink. Second, the roof may make it difficult to sense high alarm conditions and increase the risk of overfill. Either case is very costly to remediate and may also impose fines.
A high accuracy in roof position measurement may also allow for an increased accuracy of tank inventory assessment and better detection of roof landing (e.g., when the roof rests on the bottom), which can help to reduce (e.g., environmental fines) while optimizing storage capacity. High accuracy of this type is also helpful in detecting when the roof doesn't properly follow the liquid, hence provide a pre-warning before the roof becomes stuck. Also, the earlier discussed (rim) seal causes friction, which affects the immersion of the floating roof. A different immersion means that a different amount of liquid is displaced by the roof. By measuring the immersion—at one or preferably more locations, the immersion correction on the assessed tank inventory (mass or volume) can be improved.
A current solution uses radar gauges, which are disadvantageous because their measurements are unreliable as there are typically obstructions on the floating roof (e.g., which cause unwanted reflections). A highly accurate determination is difficult to obtain when measuring close to the tank wall, and measuring further away increases the installation complexity and costs.
High levels of detection of floating roofs are still mostly accomplished using ‘conventional’ mechanical switches, which are maintenance dependent and typically not subject to self-testing.